feat(telemetry): add workflow_opened with open_source and missing node metrics (#6476)

- Adds app:workflow_opened and plumbs open_source across drag/drop,
file-open button, workspace, and templates
- Tracks missing_node_count and missing_node_types for both
workflow_opened and workflow_imported
- Reuses WorkflowOpenSource type for consistency; no breaking changes to
loadGraphData callers (5th param remains options object; openSource
optional)

Validation
- pnpm lint:fix
- pnpm typecheck

Notes
- Telemetry only runs in cloud builds; OSS remains clean.
- loadGraphData telemetry is centralized where missing_node_types is
computed.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-6476-feat-telemetry-add-workflow_opened-with-open_source-and-missing-node-metrics-29d6d73d365081f385c0da29958309da)
by [Unito](https://www.unito.io)

---------

Co-authored-by: bymyself <cbyrne@comfy.org>

src/platform/telemetry/providers/cloud/MixpanelTelemetryProvider.ts

@@ -244,6 +244,10 @@ export class MixpanelTelemetryProvider implements TelemetryProvider {
     this.trackEvent(TelemetryEvents.WORKFLOW_IMPORTED, metadata)
   }
 
+  trackWorkflowOpened(metadata: WorkflowImportMetadata): void {
+    this.trackEvent(TelemetryEvents.WORKFLOW_OPENED, metadata)
+  }
+
   trackPageVisibilityChanged(metadata: PageVisibilityMetadata): void {
     this.trackEvent(TelemetryEvents.PAGE_VISIBILITY_CHANGED, metadata)
   }